Increased power density is a continuing goal of modern power supply design. High power density is particularly crucial in applications wherein the allocated space for the power supply relative to the power output is restricted. In addition to being highly compact, the power supply must also be efficient to limit heat-creating power dissipation.
In low to medium level power applications (e.g., 50 to 800 watts), a forward converter topology is widely used. A DC/DC forward converter generally includes an isolation transformer, a switch on a primary side of the transformer, and a rectifier and output filter on a secondary side of the transformer. The switch, coupled in series with a primary winding of the transformer, converts an input DC voltage into an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates therefrom a desired DC voltage that is filtered by the output filter at an output of the forward converter.
A conventional forward converter topology contains a single switch, typically a semiconductor device, sized to withstand the input voltage. Many applications, however, require input voltages that may be too high for commonly available semiconductor devices. A two-switch forward converter, therefore, has been designed to accommodate semiconductor devices rated for approximately the input voltage. The addition of a second switch on the primary side of the transformer allows a reset voltage of the transformer and the input voltage to be divided between the two switches, thus reducing voltage stresses on each switch.
A practical concern regarding forward converters is that a magnetizing current of the transformer must be taken into consideration in the design of the converter. Otherwise, the magnetic energy stored in a core of the transformer by the magnetizing current may cause a failure in the converter. To facilitate the recovery of magnetizing energy from the transformer core to the input, the two-switch forward converter incorporates first and second diodes, each coupled between a rail of the input and the primary winding. During an off state of the switches, magnetizing energy flows from the transformer core to the input via the first and second diodes, allowing the transformer core to reset.
One inherent problem with the two-switch topology is that the reset voltage of the transformer is equal to the input voltage, thereby limiting a maximum duty cycle of the switches to 0.5. By operating the switches at the 0.5 duty cycle, however, the selection criteria for the switches and the turns ratio of the transformer are limited. Obviously, these aforementioned limitations detract from the use of forward converters and raise the costs associated therewith.
Accordingly, what is needed in the art is a system and method for resetting the transformer while, at the same time, maximizing the design flexibility and increasing the efficiency of the power converter.